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Old 16th November 2007, 07:33 PM   #1
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Default Max B+ for triode-connected KT88

First post here, though I have been getting good info here for quite some time.

I am beginning to consider thinking about starting to design a push-pull triode-connected KT88 output stage. I have looked around at other's designs and noticed that B+ is usually reduced for this configuration. Is there anything wrong with a 500V+ B+ on triode strapped KT88s. Why is it not done? Or am I just not able to find it? Max Va,g2 listed in the tube data is 600V. Do current production tubes stand true to this rating?

Thanks
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Old 16th November 2007, 08:35 PM   #2
7N7 is offline 7N7  United Kingdom
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Default Re: Max B+ for triode-connected KT88

Quote:
Originally posted by SpreadSpectrum
First post here, though I have been getting good info here for quite some time.

I am beginning to consider thinking about starting to design a push-pull triode-connected KT88 output stage. I have looked around at other's designs and noticed that B+ is usually reduced for this configuration. Is there anything wrong with a 500V+ B+ on triode strapped KT88s. Why is it not done? Or am I just not able to find it? Max Va,g2 listed in the tube data is 600V. Do current production tubes stand true to this rating?

Thanks
I have done a bit of work triode-strapping pentodes and beam tetrodes.

A good start is to look at Philips's data for 6146 (QE5/20 I think). The g2 limit for this type is 250V, but Philips supplied information for a p-p triode amp running the valves at 400V.

I have tested 13E1s (g2 max 300V) triode strapped at 500V and 200mA with no apparent problems. Do you really need as much as 500V? Make sure your output transformers are up to it (they probably will be). I think that good KT88s would be OK at 500V; certainly in the thread about bullet-proof 6L6GCs Tubelab states that he runs Russian and Chinese 6L6 and 5881s at higher voltages and above maximum Pa .

Main worry at 500V for your valves is the sockets. I would try to find top-quality NOS military-style ceramic ones. Even at lower voltages in guitar amps I have had to replace bases due to tracking which caused all sorts of problems.
Good luck

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Old 16th November 2007, 11:55 PM   #3
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Default Re: Re: Max B+ for triode-connected KT88

I think that I'll try it out then, since others are exceeding the ratings for their respective tubes, unless someone posts a horror story about the KT88. I'm still more than 10% below the rating. (I'm planning on having about 530V)

I've got some new production ceramic sockets that look to be pretty good quality.

I was hoping to make something to switch back and forth between ultralinear and triode mode (AB2) output so that I could do some listening trials and learn in the process. All of the output transformers I have considered are able to handle 560V or are custom order parts.

This project is becoming quite a monster, but I'm having a blast.
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Old 17th November 2007, 01:43 AM   #4
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Quote:
Tubelab states that he runs Russian and Chinese 6L6 and 5881s at higher voltages and above maximum Pa .
Yes, I have "tested" plenty of tubes at voltages up to 550 volts. This testing was in a SimpleSE amplifier. The SimpleSE uses cathode bias which runs near 40 volts with 550 volts of B+. Including the drop across the OPT the voltage across the tube was about 490 volts. If you are planning to use cathode bias, your voltages will be similar. If you are planning to use fixed bias, your tubes will see a higher voltage.

Depending on whose specs you read the max G2 spec is 500 to 600 volts. This is the spec for the original tubes. The current Russian and Chinese tubes don't seem to have any published specs. Some live up to, or exceed the original specs, and some blow up.

Someone quoted me for "testing" Russian 5881's, but I don't have any of them. I have tested several flavors of Russian and Chinese 6L6GC's. I have also tested the Electro Harmonix KT88. They seem to laugh off the abuse that I threw at them.
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Old 17th November 2007, 06:41 AM   #5
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Tubelab,

Thank you for posting. I have already bought the electro-harmonix KT88, so it is good to hear from someone who has used them.

I'm actually in the process of constructing this amp. I am building it as a fixed bias ultralinear with a B+ of about 530V, but I am also toying with the idea of testing it out triode connected to see which sounds better. So, it would be fixed bias and would have the full 530V across the output tube. Since I already have the parts, I don't want to change the power supply too much.

I am also having crazy ideas about beefing up the driver so it can source grid current. I'll probably want to parallel some more output tubes next, who knows.
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Old 17th November 2007, 07:36 AM   #6
ttan98 is online now ttan98  Australia
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Default KT88

I had designed KT88 amp from scratch before about 10 yrs ago, back then the NEW KT88s were not very reliable. Failed regularly.

I am not so certain of the current crop of KT88s.

To be on the safe side I recommend you don't exceed 450Vdc, back then the KT88 did not last very long.

Lower B+ gives you

pro

1. better reliability
2. longer tube life
3. happier person
4. costs less to run the amp.

con

1. less power
2. slight lost of dynamics, depends on yr system though
you not notice it.

pro out-weighs the con.
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Old 17th November 2007, 09:56 AM   #7
7N7 is offline 7N7  United Kingdom
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Default Whoops

Quote:
Originally posted by tubelab.com


[...
Someone quoted me for "testing" Russian 5881's, but I don't have any of them. I have tested several flavors of Russian and Chinese 6L6GC's. I have also tested the Electro Harmonix KT88. They seem to laugh off the abuse that I threw at them.
That someone was me! Sorry; I was confusing Sovtek 6L6WXT with Sovtek 5881WXT.

As a matter of interest is there any difference?

7N7
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Old 17th November 2007, 04:40 PM   #8
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That someone was me! Sorry; I was confusing Sovtek 6L6WXT with Sovtek 5881WXT.
I haven't seen any of the Sovtek 5881WXT's, but the Sovtek 6L6WXT and the Electro Harmonix 6L6GC have a large square plate structure that looks like a KT77. These two tubes look, and test the same. It is entirely possible that the Sovtek 5881WXT is the same also. I have some old Tung Sol 5881's and they do not like my testing. They red plate at about the same power level as a 6L6GB or 807.

Quote:
I had designed KT88 amp from scratch before about 10 yrs ago, back then the NEW KT88s were not very reliable. Failed regularly.
I bought a large quantity of Chinese KT88 tubes about 10 or 12 years ago. These tubes reminded you that the Chinese invented fireworks! They would red plate at 25 watts of dissipation, and often arced over in 30 watt guitar amps that ran on 400 volts. The new ones are much better. I have been running EH KT88's in a SimpleSE for about a year at 460 volts B+ and almost 100 mA of current. No red plate or screen in a dark room, but there is a cool blue glow on the glass that dances to the bass line.
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Old 17th November 2007, 05:56 PM   #9
7N7 is offline 7N7  United Kingdom
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Quote:
Originally posted by tubelab.com


I haven't seen any of the Sovtek 5881WXT's, but the Sovtek 6L6WXT and the Electro Harmonix 6L6GC have a large square plate structure that looks like a KT77. These two tubes look, and test the same. It is entirely possible that the Sovtek 5881WXT is the same also. I have some old Tung Sol 5881's and they do not like my testing. They red plate at about the same power level as a 6L6GB or 807.

[...]

Yes I would not expect much from the original 5881s, after all they are only "industrial" 6L6GBs rated at 23W Pa. I have just ordered a pair of the Sovteks. When the arrive perhaps I'll make a snap of them and post it here so you perhaps could let me know if they resemble the 6L6GCs.

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Old 17th November 2007, 07:43 PM   #10
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Quote:
I have been running EH KT88's in a SimpleSE for about a year
How do I buy your Simple SE board?
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